Weather forecasts have helped us get information about future weather conditions for over a century. Eleven meteorological terms are especially useful in making the most sense of these forecasts.

They are:

1. 1. Low- and High-Pressure Systems
   2. Cold Fronts
   3. Warm Fronts
   4. Jet Streams
   5. Severe Weather
   6. Thunderstorms
   7. Dry Spells
   8. Wind Chill
   9. Heatwaves
   10. Tornadoes
   11. Climate Change

It is important to get to know and better understand these terms, as they will help you gain an even better understanding of how and why we are affected by different weather conditions.

Especially if you have your own personal weather station or are a weather enthusiast keen on better understanding how everything fits together, the following information will be especially useful.

We are going to discuss the 11 terms listed in more detail since they make up some of the most important and relevant weather terms and weather phenomena. 

We start with some weather terms that many of you may already be familiar with:

Low- and high-pressure systems are probably among the most well-known and widely used terms in meteorology. We first need to understand what air pressure is before we can delve into low- and high-pressure systems.

Air pressure is defined as the weight of the air molecules in a specific space pushing down on the surface below as a result of the Earth’s gravity.

It can be seen as the number of molecules present in a certain volume of air at any given moment or during a specific period of time.

Air pressure is created in a variety of different processes, which in turn leads to different changes in weather. This leads us to low- and high-pressure systems.

A low-pressure system is defined as a specific area where the weight of the air (or the number of molecules present in this volume of air) is lower than that of the air in the surrounding areas.

The process through which a low-pressure system is formed is called cyclogenesis, which is the umbrella term for the different circulation processes involved in the formation of a low-pressure system.

(We don’t need to look at all of them for the purpose of this article, though. Just know that the term “cyclogenesis” is representative of all these different processes.)

Development

For a low-pressure system to form, certain conditions must be present to cause a drop in air pressure. A low-pressure system is normally formed as a result of 2 different processes:

1. 1. Wind Divergence Aloft
   2. Thermal Lows

1) Wind Divergence Aloft

Wind divergence aloft causes air in the upper troposphere to move apart, encouraging air at the surface to rise.

(This creates an area of lower pressure aloft, which encourages air from below to rise.)

Low-pressure systems that form as a result of this effect mainly develop in 2 areas:

1. 1. To the east of upper troughs (which normally have long wavelengths)
   2. In front of shortwave troughs

2) Thermal Lows

Many of you familiar with tropical depressions (and the resulting tropical storms, hurricanes, cyclones, etc.) will be familiar with the way in which these low-pressure systems are formed.

As the surface of our oceans and landmasses is warmed by the sun, it causes the air above it to heat up as well. The warmer air starts to rise, leaving less air at the surface, which causes a low-pressure system to form.

Characteristics Of A Low-Pressure System

Low-pressure systems are almost always associated with cloudy and rainy weather. (You only need to look at any weather forecast to notice how often a low-pressure system is mentioned in the same breath as cloudy & rainy conditions.) There is a good reason for this.

As the air above the low-pressure system continues to rise, it starts to cool down. As the moisture-carrying air cools down, condensation and cloud formation take place, which normally results in precipitation.

As air normally flows from an area of high pressure to low pressure, winds tend to blow inwards towards the area of low pressure.

This inward circulation of air is influenced by the Earth’s rotation. This effect is more commonly known as the Coriolis Effect. As a result, the winds rotate clockwise around a low-pressure system in the Southern Hemisphere and counterclockwise around a low-pressure system in the Northern Hemisphere. (Also referred to as cyclonic flow.)

A high-pressure system can be defined as a body of air whose weight (or amount of molecules present in the volume of air) is more than that of the air in the surrounding areas.

Development

In many ways, a high-pressure system and its development can be seen as the direct opposite of a low-pressure system. This is especially evident in the way it is formed…

Warm air that has risen from the equator cools down, and the resulting precipitation dries out the air, which then starts moving towards the poles.

The cold, dry air from the upper troposphere starts descending (as the cooler air weighs more than the surrounding warmer air). The air converges at the top of the high-pressure system, strengthening the descent of the cooler air.

As the cool air continues to descend, it starts compressing as it nears the surface. This results in a high-pressure system forming where the descending air reaches the centre of the weather system.

Characteristics Of A High-Pressure System

High-pressure systems are normally associated with clear sunny weather and light surface winds. (The cool, dry air, combined with the air heating up as it descends, inhibits cloud formation and precipitation.)

As air always moves from an area of high pressure to low pressure, winds blow outwards and away from the center of the high-pressure system.

Just like low-pressure systems, the winds rotating around a high-pressure system are influenced by the Coriolis Effect (caused by the rotational spin of the Earth).

Unlike low-pressure systems, though, the winds rotate in a clockwise direction around high-pressure systems in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.

A cold front occurs when the leading edge of a mass of cold air moves into a region of warmer air. The boundary between these two air masses is called a cold front. A cold front is normally associated with wet and stormy weather conditions.

This characteristic stormy weather that accompanies a cold front is a direct result of the “collision” between the 2 air masses. As the edge of this fast-moving cold air mass reaches the warmer air, the cold (and heavier) air undercuts and starts lifting the warmer (lighter) air up into the atmosphere.

As the warm air is lifted, it starts to cool down, causing moisture in the air to condense into tiny droplets. If enough moisture is present in the rising air, this will lead to cloud formation and precipitation.

Please note that this is a very broad description of a cold front. For example, a cold front that meets and lifts a body of warm air that carries little or no moisture may not display any of the normal characteristics associated with a cold front (e.g., rain, clouds, and wind).

Many different weather systems can be responsible for the formation of a cold front. We will discuss this in more detail in another article.

A warm front occurs when the leading edge of a mass of warm air moves into a region of colder air. The boundary where these two air masses meet is called a warm front. 

A warm front is normally associated with slow-moving stratus-type clouds producing light rain for a sustained period of time. (Similar to a stationary front.)

As the warm air cannot replace the denser and heavier body of colder air, it is forced to rise and move over the boundary of the colder air mass. This process is called overrunning.

If there is enough moisture in the air (which is not always the case), the rising warm air will start to cool down as it moves up and over the mass of cold air. As a result, condensation and cloud formation will occur, normally producing sustained light rain, often followed by a light drizzle later on.

As a warm front moves much slower than the more disruptive cold front, the weather changes associated with it are also more prolonged and not that severe. It is normally preceded by high-forming clouds that slowly get replaced by lower cloud formations as the warm front approaches.

The actual arrival of the warm front is normally accompanied by a drop in air pressure. The light rain previously mentioned normally arrives with the warm front, and as the front passes over, it turns into a light drizzle.

As in the case of a cold front, the conditions described above are fairly broad characteristics of weather normally associated with a warm front. (As a result, a variety of different weather conditions can occur due to specific characteristics of a warm front, as well as the part of the world it occurs in.)

Jet streams are defined as long, narrow bands of strong winds, blowing at high velocities above the Earth’s surface, normally found in the upper troposphere at heights of 9 to 16 kilometers (30 000–52 000 feet) above sea level.

There are 4 major jet streams present above the Earth’s surface, and they all have a huge influence on the global climate and the formation of various weather systems.

The 2 strongest jet streams are found near the South and North Poles, respectively, at heights of 9–12 kilometers (30 000–39 000 feet). The 2 weaker subtropical jet streams occur at a height of 10–16 kilometers (33 000–52 000 feet).

Jet streams are mainly formed as a result of 2 processes.

1. 1. Solar Radiation (the heating up of the atmosphere), resulting in the influential Hadley, Polar, and Ferrel circulation cells
   2. The Coriolis Effect (a result of the Earth’s rotation, affecting global air movement)

Jet streams do not always move in a straight line, but rather in a more meandering manner as they move between areas of hot and cold air, forming a boundary between these pockets of warmer and colder air.

Jet streams are also influenced to a large extent by the difference in temperature between these areas of hot and cold air. A bigger difference in temperature between the warmer and colder air masses will result in a substantial increase in the velocity at which the jet stream is traveling.

Jet streams are especially important to the aviation industry and closely monitored by major players in the industry. Using jet streams flowing in the same direction a plane is travelling is beneficial for saving fuel and reaching a destination on time (or making up lost time).

(Get it wrong, however, and airlines may end up with their planes flying directly into an opposing jet stream flow, leading to additional fuel being used and flights potentially arriving late at their destination.)

Severe weather refers to any meteorological phenomena that are dangerous and potentially destructive. These can lead to severe damage, disruption of large areas of infrastructure, and even loss of life. This includes thunderstorms and lightning, hail, heavy rain & flooding, tornadoes, and severe wind conditions.

Depending on where on the planet you are situated, you will be affected by severe weather conditions that are common to that specific region.

For example, areas in India may be very susceptible to flash flooding due to the seasonal summer monsoon bringing with it huge amounts of rainfall.

Similarly, if you live in Tornado Alley (the area found in the Great Plains of the Central United States), you are more likely to suffer the devastating effects of tornadoes forming during the spring and summer months.

No matter where you find yourself, it is always important to pay attention whenever severe weather warnings are issued. You and your family’s lives may literally depend on it.

Thunderstorms are violent and fairly short-lived disturbances in atmospheric conditions, normally associated with lightning and thunder, strong winds, heavy rain, and even hail. They are normally associated with, and a result of, a sudden buildup of cumulonimbus clouds.

For thunderstorms to occur, three key ingredients need to be present:

1. 1. Moisture
   2. A Lifting Mechanism (normally in the form of heat)
   3. Atmospheric Instability

With these 3 ingredients present, a thunderstorm will go through three phases of development to complete the process:

1. Developing (Cumulus) Stage: Warm, moist air near the surface starts to rise. As it continues to rise, it cools down, and the moisture in the air condenses and forms micro-droplets. If the air is unstable enough and continues to rise, it leads to the formation of cumulus clouds. Through a process of convection, the air is driven higher into the atmosphere by updrafts.
2. Mature Stage: The air continues rising until it reaches a region of warmer air, which stops it from rising any further. It starts spreading out horizontally, and large amounts of moisture combine to form larger droplets. As they start falling, they create downdrafts which, together with the continuing updrafts, produce the mature cumulonimbus cloud. This causes severe internal disturbances within the cloud, resulting in the severe conditions we commonly associate with thunderstorms.
3. Dissipating Stage: At this stage, a process called a downburst can occur as the downdrafts overwhelm any more updrafts and air inflow into the thunderstorm. This process happens very rapidly, as this downburst carries air quickly to the ground and then spreads out, after which the thunderstorm starts to dissipate relatively quickly.

You also get more than one kind of thunderstorm. Single-cell thunderstorms, multi-cell clusters, and supercells are just a few well-known examples. (We will discuss these different types of thunderstorms in a separate article.)

There is one more important fact to take note of. Even though most thunderstorms look violent and spectacular, in order for them to be officially classified as severe thunderstorms, they need to meet one or more of the following criteria:

• • Wind speeds of at least 93 kilometers per hour (58 mph)
  • Hail with a diameter of 25 millimeters (1 inch)
  • The presence of tornadoes

A dry spell can be defined as a sustained period of dry weather with lower water and soil moisture levels due to a lack of rainfall. A region with significantly lower rainfall figures during its rainy season compared to previous seasons can be regarded as experiencing a dry spell.

A dry spell should not be confused with a drought, however. There is much debate and confusion about the difference between the two, and depending on which region you find yourself on the planet, definitions may vary.

In general, a dry spell does not last as long as a drought. Although it puts a strain on natural resources, especially in the agricultural sector,  it normally does not pose an immediate threat to human or animal life.

A drought, on the other hand, is a much more serious condition with severe consequences. Probably the most important feature setting it apart from a dry spell is the length of time over which it occurs.

It can last through multiple dry spells, sometimes taking years or decades to fully develop. The results are normally devastating. Water supplies and resources that depend on them can be completely depleted.

Often, this results in the destruction of the agricultural sector, the lifeblood of any country or region. This will directly threaten the livelihood of the region’s inhabitants and cripple the sustainability of all processes necessary for growth and survival.

As a result, one should be very careful about referring to sustained dry spells putting a strain on any region for a period of time as a drought.

(Something the news media is sometimes quick to jump on for dramatic effect, often causing undue panic in the process.) They may have similarities, but they are very different in their extent and level of seriousness.

Wind chill (or wind chill factor) refers to the phenomenon where you experience temperatures around you as much colder than they actually are due to the presence of wind. It is caused by wind increasing the rate at which your body loses heat, causing you to perceive temperatures as colder than they actually are.

You are experiencing the temperature to be that much colder because your body’s natural heat creates a layer of warm air around your skin to provide a form of insulation from the surrounding cold air. Wind blowing against your skin, however, removes this insulation layer, making you experience the temperature as much colder.

When you hear or read the term “feels like…” next to the temperature given, it may refer to the wind chill “temperature,” in case you were wondering.

A heatwave refers to a period of prolonged exceptionally hot weather, often accompanied by high humidity levels. It is often determined by comparing temperatures to the region’s average hottest temperatures during the same period in previous seasons.

The precise definition differs between regions and weather services. Sometimes this can lead to much confusion. For example, in certain parts of Australia, a heatwave is defined by 5 consecutive days of temperatures exceeding 35 °Celsius (95 °Fahrenheit) or 3 consecutive days of temperatures exceeding 40 °Celsius (104 °Fahrenheit).

The South African Weather Service, on the other hand, defines a heat wave as the maximum temperature in a specific area being 5 °Celsius hotter than the average maximum temperature of the hottest month of that specific area for at least 3 consecutive days.

As you would have noted from the 2 samples above, definitions can vary widely from one region to another. Rather than relying on specific numbers, the definition highlighted in bold at the start of this section should be considered to be a more accurate determination of a heatwave in your area.

Heatwaves are a result of the formation and strengthening of high-pressure systems in the upper atmosphere (3 000–7 600 meters or 10 000–25 000 feet). As weather patterns often move more slowly during the summer months compared to winter months, they can linger over a specific area much longer.

The air under the high-pressure system dries and warms as it is forced down and sinks towards the surface. This, in turn, forms an inversion layer, preventing convection from taking place and trapping the hot, humid air beneath it.

You can learn more about a heatwave in this article.

A tornado is a funnel-shaped, rapidly rotating column of air. Tornadoes normally form at the base of cumulonimbus clouds and can cause various degrees of damage, depending on wind speeds and their intensity.

Tornadoes are classified on a scale from EF0 (the weakest form, doing minimal damage) to EF5 (the strongest form, able to rip houses clean off their foundations and do significant damage to infrastructure).

Wind speeds are normally around 180 km/h (110 mph) or less, but in Category EF5 storms, wind speeds of over 450 km/h (300 mph) can be reached with catastrophic consequences. Tornadoes are also around 250 meters (820 feet) in diameter but can be as big as 3 kilometers (2 miles).

Tornadoes are formed when strong updrafts and downdrafts interact beneath a thunderstorm, helping to create and strengthen rotating air. As the updrafts intensify, an area of very low pressure can develop and extend towards the surface.

This results in the familiar funnel cloud, with the section reaching the surface containing very strong winds and rapidly rotating air, leading to the severe damage often associated with strong tornadoes.

You can find in-depth information about tornadoes in this article.

As you will be very well aware, climate change is a very controversial topic and a subject for a whole series of articles on its own. Since it has been widely used over the past few decades in the same breath as global warming (with its different causes and its effect on the environment and the planet as a whole), an explanation of this term is warranted.

In a nutshell, climate change refers to long-term changes in the state of the Earth’s climate over a period of at least 3 decades or more. This includes global temperature changes, the flow of our oceans’ currents, and rainfall, to mention a few variables.

Earth’s climate has gone through natural cycles of warming and cooling, including ice ages, over millions of years.

What makes climate change that much more relevant and important to us is the unusually rapid pace at which it is taking place now and its close ties to human activity since the Industrial Revolution, particularly through the continued burning of fossil fuels and release of greenhouse gases into the atmosphere.

But this is a topic for a whole other conversation.

Eleven very important weather terms were addressed in this article. Hopefully, this will clear up many elements of the weather you were unfamiliar with.

You will note that some terms received a lot more attention and were explained in more detail than others. Those are the terms I regard as especially important to understand, as they will help you better understand many other weather terms and occurrences.

Until next time, keep your eye on the weather!